WASHINGTON — Remarkable for its last-minute surge of contributions, the U.S. presidential election witnessed unprecedented sums of cash boosting two men in their quest for the White House. It was a cost that surpassed $2 billion and sometimes came with the cloak of anonymity for billionaire donors.

The election was the first in which “super” political action committees spent hundreds of millions on television ads, especially those supporting GOP presidential nominee Mitt Romney. Super PACs, like those helping President Barack Obama, benefited from deep wells of money from wealthy donors and corporations.

A handful of mega donors stood out. The most prominent were Las Vegas casino mogul Sheldon Adelson and his wife, Miriam, who together contributed nearly $100 million — as promised — to help Republican candidates. On the left, celebrities like Jeffrey Katzenberg poured millions of dollars into efforts helping Obama win a second term.

More than $230 million in super PAC money bolstered Romney’s candidacy, adding to the massive haul by the Republican Party for the former Massachusetts governor. The pro-Romney super PACs were able to hammer the president in swing states with meticulously designed ads highlighting a woeful economy and what they portrayed as Obama’s failed record.

A sizable chunk of that cash flowed in just weeks before Election Day. Because Federal Election Commission rules don’t require groups to report until late November the money they’ve raised since mid-October, many top donors escaped scrutiny until after the Nov. 6 voting. The Adelsons’ $33 million gift to two pro-Romney super PACs, as well as $3 million from Larry Ellison, head of software giant Oracle Corp., were not divulged until Thursday.

The pro-Obama Priorities USA Action raked in nearly 20 percent of the money it raised this election during the final weeks of the campaign. Much of that $15 million haul, records show, came from repeat million-dollar donors like Fred Eychaner, the founder of Chicago-based Newsweb Corp., and from the ranks of Renaissance Technologies, whose investors donated $4 million in the campaign’s final weeks.

Those pots of money, in turn, enabled super PACs to dole out millions of dollars on pricey television ads in important swing states, including some where razor-thin ballot margins had been forecast for Election Day.

“The super PACs helped Romney run a more competitive race,” said R. Donahue Peebles, an Obama fundraiser from New York. “But, in the end, money can take a candidate only so far.”

Surpassing the $2 billion record was long expected after an election season dominated by the supercharged competitive pressures that both campaigns faced in mounting massive fundraising blitzes to stoke expensive media ad battles and ground wars. The Obama and Romney campaigns mobilized competing squads of ultra-wealthy fundraisers, sought aid from free-spending allied super PACS and deployed multimillion-dollar media broadsides and armies of organizers.

Romney and the GOP reported raising more than $920 million by election’s end, compared with Obama and the Democrats’ $960 million. Obama had been largely outspent by Romney and allied groups during the summer, but the president’s campaign began to close that gap as Election Day approached.

Campaigns and outside groups brought in more than $1 billion to help each candidate, an Associated Press review of financial records showed. In 2008, Obama shattered records by raising more than $750 million in donations. Romney’s campaign, for its part, said it stretched its dollar competing against an incumbent.

“Every dollar we raised was put to use in the effort to elect Mitt Romney,” said finance chair Spencer Zwick, citing strong fundraising during the final weeks leading up to Nov. 6. Romney’s election effort brought in $85.9 million since mid-October, compared with Obama’s $111 million during the same period.

After a series of high-profile federal court rulings, the nation’s relaxed campaign-finance system allowed for unlimited contributions from corporations, labor groups and others; television advertisements from nonprofit groups that concealed who paid for them and the proliferation of more than 1,000 super PACs. Those groups can’t coordinate with the candidates they support, but groups on both sides of the political aisle were staffed with former campaign advisers who were deft political fundraisers.

But the election was known just as much for its sources of so-called dark money as it was for its hefty price tag.

Nonprofit “social welfare” organizations spent hundreds of millions more on so-called issue ads, and those groups don’t have to disclose their donors because they’re governed by tax law. Open-government groups have pushed Congress, to no avail, for a law that would require politically active groups to reveal their finances.

As well, federal rules require timely disclosure for super PACs, but determining who’s behind big donations isn’t always easy. In summer 2011, a fledgling company dissolved shortly after making a $1 million contribution to a super PAC supporting Romney; records showed that the company, established and closed over a four-month period, was formed by a Romney supporter who once worked with him at the private equity firm Bain Capital.

Other super PACs active this election season benefited from opaque, eleventh-hour contributions. FreedomWorks for America, a prominent tea party group, reported more than $5.2 million in donations during the first half of October — about 90 percent of the group’s fundraising haul — from an apparent shell company in Knoxville, Tenn., called Specialty Group that advertises no product or service.

The company’s owner, William Rose, said in a statement he was under no obligation to reveal where his money — ultimately used to boost high-profile congressional races — came from.

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